Tetrahedral metal-peptide cage with flexible scaffolds for induced-fit encapsulation

To construct metal-organic cages (MOCs), rigid π-extended aromatic molecules have been considered necessary as the backbone. Here we report the construction of a novel metal peptide cage using oligoproline, a type of peptide, as a scaffold instead of an aromatic molecule. The oligoproline ligand bearing both terminal bipyridyl groups complexed with Fe2+ ions spontaneously to form a M4L6-type tetrahedral cage preferentially. This cage could capture and encapsulate a middle molecule drug, cyclosporin A, in water via adapting its inner cavity to the guest molecule, which is similar to a substrate uptake by enzyme. This study paves the way for the development of a new class of MOP, hopefully leading to flexible host materials including drug delivery systems and artificial enzymes.